**Proficiency Bonus is never added to damage rolls. Period.** 

Proficiency bonus is only added to attack rolls, skill rolls, and saves for which you are proficient. Damage rolls will only ever add the appropriate modifier for the stat which was used to make the attack. However many spells, especially cantrips, do not add your casting mod to the damage. 

**Firebolt requires you roll 1d20+INT vs. AC and deals 1d10 damage** 

>Make a ranged spell attack against the
target. On a hit, the target takes 1d10 fire damage. A
flammable object hit by this spell ignites if it isn't being
worn or carried. - PHB p. 242

**Spell attack = roll a d20 and add prof and mod. Spell save means target rolls defense.**

Spell attack means its going to be Casting Mod + Proficiency bonus (if you are using the correct implement)  vs. Target AC. 

Spells that require a save on the other hand require no rolls from you to attack. You may need to roll damage (such as for an AoE spell like burning hands) and then the target would roll their save vs. your spell save dc (8 + prof + casting mod). 

**There are two types of attack spells requiring d20 rolls, melee attack spells and ranged attack spells.**

The difference is that melee attack spells will not be made at disadvantage when next to an enemy combatant whereas ranged spells would be (and all ranged attacks for that matter).